Prescribed medical cannabis isn't an immunity card. A medical defence exists...

https://direct-wiki.win/index.php/What_Happens_If_I_Crash_and_THC_is_Found_in_My_Blood%3F

Prescribed medical cannabis isn't an immunity card. A medical defence exists for the 2mcg/L THC limit, but DVLA fitness rules still apply. Legality doesn't replace responsibility; if you’re impaired, you shouldn't be driving. Stay safe and aware.